DEVELOPMENT AND CHARACTERISATION OF NOVEL FLUORAPATITE GLASS-CERAMICS FOR USE IN GLASS IONOMER CEMENTS
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Introduction: Remineralisation potential of GICs can be improved by using apatite additives in the cement. The aesthetics of GICs depends on refractive index match between the different components in the material. The problem arises when the cement with the filler is mixed because apatite particles in the cement paste agglomerate during the mixing process and scatter light at the interfaces as a result of RI mismatch. The RI of the fluoride-containing GICs glasses is also difficult to predict and match to the polysalts/liquid component. Aims: 1. to modify Appen model so that the refractive indices of bioactive glasses and the fluoride-containing GIC glasses can be calculated to match the refractive index of the liquid component and the polysalts produced during the setting reaction; 2. to develop strontium-containing optically clear FAp glass-ceramics with known refractive indices. Methods: Refractive indices of ionomer-type and bioactive glasses were measured which were then used to derive Appen factors for the amorphous metal fluorides. Subsequently, using the derived Appen factors, two series of glasses were designed based on the SiO2-Al2O3-P2O5-CaO/SrO-CaF2/SrF2 components. The developed series were then analysed for their ability to crystallise to apatite phases by DSC, XRD, ATR-FTIR and MAS-NMR solid-state characterisation techniques. Results: It was found that Appen factors for the metal fluorides are higher than the RI of the corresponding crystal phases. It was also found that the fluoride content and the RI of GIC glasses correlate linearly. It was found that the developed glasses with predicted RIs crystallised to apatite phases on controlled heat-treatment. More detailed analysis of the developed glass-ceramics revealed this to be in the form of acid-resistant fluorapatite. Increasing strontium content in the developed base glasses was found to promote surface crystallisation and resulted in a linear increase in strontium incorporation into the FAp lattice. Conclusions: Appen model was successfully modified which can be used to predict refractive indices of fluoride-containing glasses used in dental cements. RI-tailored nanoscale FAp glass-ceramics have been successfully developed for use in aesthetic GICs. Increasing strontium content in the developed compositions was found to promote surface crystallisation of FAp phases. Developed FAp GCs can be incorporated into GICs for improved remineralisation.
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